A 25-year-old woman presents with symptoms suggestive of an acute exacerbation of asthma: increasing shortness of breath, wheeze, and cough over the past 24 hours. Her peak expiratory flow rate (PEFR) is 60% of her best. She is using her salbutamol inhaler every 2 hours. According to NICE/BTS guidelines, what is the most appropriate immediate management in primary care?